1. Nicotinic acid hydroxylase (NAH) from Clostridium barkeri and formate dehydrogenase H (FDH) from E. coli are selenium-containing enzymes. Our previous EPR studies of Se(77)-enriched NAH and FDH demonstrated that selenium is directly coordinated to molybdenum(V) ion of these Mo- containing enzymes. NAH incorporates Se in an unidentified dissociable form, whereas FDH contains selenocysteine (Se-cys 140). The Se-cys 140 is located in a conserved amino acid sequence characteristic of other procaryotic molybdopterin-containing enzymes. In these enzymes the site corresponding to Se-cys140 is occupied by cysteine or serine. The homology between FDH and other procaryotic molybdopterin-containing enzymes allows us to identify this sequence as a domain (of accounted for coordination of Mo) spanning the Mo coordination site. 2. ENDOR investigations provide structural information about protein nitrogen and hydrogen atoms, as well as phosphorous atoms of the pterin cofactor that are in the vicinity of the Mo center. Parameters and symmetry of g-, A[Mo(97)]-, A[Se(77)]- and A[P(31)]-tensor and comparison with data available for other Mo-enzymes and model compounds imply that the Mo(V) ion appears to have a short Mo=O (z-axis) bond perpendicular to the base of the square pyramid. The Mo atom is located near the center of the base (x,y-plane) and surrounded by one Se atom, two S atoms of the pterin and one unidentified ligand with nucleus spin I=O. In the case of NAH, estimation of the distance between Mo(V) and the P atom of the pterin cofactor as 8 A indicates that the Mo-P axis is almost coincident with Mo=O bond. To provide the axial position for the P-atom a bent conformation of the molybdopterin should be assumed. The ENDOR data indicate that in FDH the P-atom of the Mo-pterin is close to the x,y-plane.